1. Field of the Invention
The invention relates to a process for preparing dispersion powders by spray drying of aqueous polymer dispersions in a nozzle atomization dryer.
2. Description of the Related Art
Dispersion powders are polymer powders which are obtainable by spray drying of the corresponding aqueous polymer dispersions. A distinction is made between dispersion powders which are redispersible in water, and those which do not have this property. Water-redispersible polymer powders are obtained generally by drying of the corresponding aqueous polymer dispersions, in the presence of a drying aid (generally a protective colloid), optionally, of an antiblocking agent and, optionally, further additives. The protective colloid fraction on the one hand prevents irreversible sticking of the polymer particles to one another during the drying operation, the polymer particles being enveloped by the protective colloid particles. On the other hand, this protective colloid matrix, which dissolves again when the polymer powder is dispersed in water, has the effect that in the aqueous redispersion, the polymer particles are present again with the particle size of the original dispersion (TIZ-Fachberichte, 1985, Vol. 109 (9), 698).
In the case of dispersion powders which are not to be redispersible, it is possible optionally to do without the addition of protective colloid at the drying stage.
In order to minimize instances of powder blocking, particularly in the case of dispersion powders based on polymers having a glass transition temperature Tg of ≦25° C., and in order to improve pourability and flowability, antiblocking agents (anticaking agents) are added to the dispersion powders during their preparation. The antiblocking agent may be added during the drying step, or after drying.
The antiblocking agent (ABA) serves to ensure the pourability and the storage stability of the polymer powder obtained after spray drying (dispersion powder). Antiblocking agents are a constituent of the dispersion powder, particularly when a low glass transition temperature of the polymer, or severe hygroscopy, would lead one to anticipate blocking of the powder or of caking on the dryer wall. The powdering of the particle surface with antiblocking agent also has the particular effect, owing to reduced agglomeration, of reducing the fraction of large granules, and this may have very advantageous consequences for the drying operation and also for the properties of the powder (e.g., pourability and storage stability).
Dispersion powders of these kinds are used in a multitude of applications, including their use in coating materials and in adhesive materials for a very wide variety of substrates. One example of the use of dispersion powders which are not water-redispersible is their use as binding powders for particulate natural materials (e.g. Vinnex® powder from Wacker Chemie AG).
Water-redispersible dispersion powders are in widespread use in chemical construction products, frequently in conjunction with mineral binders (Vinnapas® powder from Wacker Chemie AG). Examples of these products are construction adhesives, especially tile adhesives, renders, and mortar compositions, paints, filling compounds, leveling compounds, thermal insulation composite systems, and jointing mortars. The advantage of the water-redispersible dispersion powders lies in particular in the possibility of using them in prefabricated, storable dry mixes, optionally together with mineral binders such as cement, and of rendering these mixes ready for use by addition of water only immediately before use. Dry mixes of these types are easier to transport than systems in paste form, since the dry mixes have no water fraction. The dry mixes also provide advantages in the context of storage, such as insensitivity to frost and resistance toward microbial infestation, for example.
Dispersion powders are produced in known spray dryers, which can be divided into nozzle atomization dryers and rotary atomization dryers. In the case of nozzle atomization dryers, the polymer dispersion (feed), generally at the top end of the dryer, is atomized under pressure, by means of one or more single-fluid nozzles (pressure nozzles), or is atomized together with pressurized air via one or more two-fluid nozzles or one or more multifluid nozzles. In the case of rotary atomization dryers, the atomization takes place by means of rotary atomizers, in the form of an atomizer disk, cup atomizer, or porous hollow cylinder, for example.
The general procedure is to supply the polymer dispersion to the spray dryer without preheating, at ambient temperature, and to dry it with a hot drying gas. In general the entry temperature of the drying gas is in the temperature range from 100° C. to 200° C. This approach has already been described in numerous instances, as for example in DE 3101413 C2, DE 4406822 A1, DE 19752435 A1, EP 407889 A1, EP 1000113 B1, EP 1110978 B1, EP 1720933 B1, EP 2341084 A1, and WO 01/83071 A1. The reason for this is that aqueous polymer dispersions having a low minimum film-forming temperature (MFFT) of below 60° C., in other words a MFFT typical of aqueous dispersions of thermoplastic polymers, are considered to be very sensitive at high temperature. The same is true of dispersions of emulsion polymers having a low glass transition temperature Tg. For these reasons, therefore, EP 332067 A1 advises spraying the dispersion together with water.
DE 10 2011 080 233 A1 describes a process for preparing dispersion powders wherein the drying gas is preheated to an entry temperature of 130° C. to 210° C. in order to accelerate drying. The polymer dispersion to be dried is preheated to a temperature of 50° C. to 98° C. and is sprayed using rotary atomizers. Chemie Ingenieur Technik 2009, 81, No. 6, 699-716 advises a flash process for temperature-insensitive substances on spraying using single-fluid nozzles, the suspension to be sprayed in this process being heated to a temperature of 120° C. to 200° C.